1. Field of the Invention
The present invention relates generally to integrated circuit fabrication, and more specifically to a method for fabricating interlevel contacts.
2. Description of the Prior Art
Aluminum is used extensively in the field of semiconductor integrated circuit fabrication for providing interconnect between various portions of an integrated circuit chip. Aluminum has several important advantages which make it the conductor of choice for many applications. Among the properties which make aluminum so useful is the fact that it is very conductive, it forms a good mechanical bond with various dielectric layers generally used in a semiconductor industry, and it makes a good ohmic contact with both N and P type semiconductors.
Aluminum also has several important drawbacks which must be overcome when fabricating integrated circuits. Aluminum forms fairly low temperature alloys. As a result, its rate of self-diffusion can be significant at temperatures which fall within the expected operating range of the semiconductor circuits in which it is included. This movement of aluminum atoms is especially severe if the aluminum interconnect carries relatively high current densities. This phenomenon is known as "electromigration," and can cause premature failure of semiconductor devices. To prevent failures due to electromigration, semiconductors must be designed so that the current density in aluminum lines does not become enough to cause rapid electromigration.
Electromigration problems can become especially severe when aluminum interconnect lines cross abrupt height changes on the surface of an integrated circuit. Such abrupt changes are generally referred to as steps. Thinning of aluminum interconnect lines tends to occur over such steps, increasing current density at these locations and making the device more susceptible to electromigration problems.
Step coverage problems are common when small openings are made through a relatively thick dielectric layer to allow contact with conductive regions beneath. These problems are especially severe with sub-micron device geometries. Contact openings, also referred to as vias, can be made to contact an underlying active region within a semiconductor substrate, or to contact an underlying polycrystalline silicon or metal interconnect layer. The sputter deposition or evaporation techniques used to form aluminum thin film layers produces a much thinner layer of metal along the edges of such an aperture due to a self-shadowing phenomenon. The higher current densities found in the thinner sidewalls can contribute to significant electromigration problems for an integrated circuit device.
Present techniques for minimizing step coverage problems through small apertures include creating apertures having sloped sidewalls and filling the contact opening with a refractory metal alloy. Creating sloped sidewalls on the contact openings increases their overall size, which limits device density. This is especially important for submicron devices. Filling the contact opening with a refractory metal alloy adds significant complexity to present process flows. Both of these techniques also tend to result in an uneven upper surface, making it difficult to stack contacts one above another.
Another proposed technique for filling contact openings is to deposit a thin layer of aluminum at a low temperature, substantially room temperature, followed by deposition of a thick aluminum layer at 400.degree. C. or higher. Such an approach is believed to be difficult to perform, and results in contact openings which are still not completely filled. A thick aluminum layer must be used, and spiking problems can occur.
It would be desirable for a technique for manufacturing integrated circuits to provide for completely filling contact openings with a conductive material. It would be further desirable for such a technique to be simple and compatible with current processing technology.